(a) Field of the Invention
The present invention relates to an objective lens system for endoscopes having a function to vary focal length.
(b) Description of the Prior Art
In the recent years where it is strongly desired to observe magnified images through endoscopes, there have been developed objective lens systems which can be focused on objects located at short distances. For example, Japanese Examined Published Patent Application No. 15004/55 discloses an endoscope which comprises an objective lens system having a retrofocus type composition consisting of a negative front lens group and a positive rear lens group, and is so adapted as to perform focusing by shifting the positive rear lens group. This conventional example of an objective lens system for an endoscope can be focused continuously in a range from far object point to near object point. However, since an endoscope, especially a medical endoscope, requires a variety of operations such as air feeding, water feeding and angle operation other than the focusing operation, an endoscope permitting continuous focusing cannot always perform focusing effectively. Further, it is conceivable to design an optical system which has a field angle wide enough to observe a broad range within a short time so as not to miss abnormal locations and which allows observations in two conditions, i.e., a condition where the objective lens system is focused on an object at such a distance at which an adequate depth of field is kept and the other condition where the objective lens system is focused on an object located at a shorter distance for observing a magnified image of a found abnormal location.
Even when such an optical system is used, however, the focal length for observation of a magnified image is scarecely different from the focal length for ordinary observation. Taking into consideration the necessity to shorten focal length to widen field angle for the ordinary observation, it is impossible to obtain a high magnification level for a short object distance since the objective lens system has a short focal length even when the objective lens system is brought rather close to an object, and the endoscope must be brought nearer the object to obtain a sufficiently high magnification level. In a case where the root of the distal end of an endoscope is bent for observation, for example, sophisticated operating techniques are required to bring the distal end of an endoscope close to an abnormal location while keeping the abnormal location within a visual field even when the abnormal location is found in the ordinary observing condition. Further, due to the limit inherent in the structure of endoscopes, it is often impossible to bring the distal ends of endoscopes close to certain locations which can be observed from certain distances. Therefore, the optical system described above is insufficient for searching for abnormal locations within short times and observing magnified images of found abnormal locations for accurate diagnosis.
In contrast, the endoscope disclosed by Japanese Examined Published Patent Application No. 44283/61 comprises an objective lens system which permits observing objects located at certain long distance at a sufficiently high magnification level since said objective lens system consists of three positive, negative and positive lens groups, and is so adapted as to perform prolonging of focal length while the lens system is focused on an object located at a short distance by shifting the negative lens group.
However, this conventional example is designed as a retrofocus type by dividing the positive first lens group into two positive and negative lens groups in order to obtain a wide angle when the lens system is focused on an object located at a long distance. Accordingly, this lens system is substantially composed of four lens groups, and has a drawback that it has a complicated composition or comprises a large number of lens components.
Further, the objective lens system disclosed by Japanese Unexamined Published Patent Application No. 193512/58 is so designed as to perform zooming and focusing separately. As a result, this objective lens system makes operations of an endoscope combined therewith undesirably complicated, and undesirably thickens outside diameter of the distal end of the endoscope since the objective lens system requires two movable mechanisms in the distal end of the endoscope. From both the functional and structural viewpoints, it is desirable to design a single movable mechanism as a vari-focal mechanism of an objective lens system for endoscopes which can establish a focal length and a focus point at the same time desired for observation.